Semiconductor laser devices such as ridge waveguide lasers and laser amplifiers are used in many communications systems. Incremental refinements in their fabrication and packaging have resulted in a class of devices that have acceptable performance characteristics and a well-understood long-term behavior. Moreover, weakly guiding ridge waveguide structures are less complex to fabricate and provide excellent yields as compared to more complex architectures based on buried heterostructures, for example.
In most applications, maximizing the laser""s or amplifier""s useful operating power is a primary design criteria. In signal laser applications, the power output from the device dictates the distance to the next repeater stage, and the number of stages in a given link is a major cost factor in the link""s initial cost and subsequent maintenance. In pump laser applications, where typically multiple pump laser devices are used to optically pump a gain or amplifying fiber, such as a rare-earth doped fiber amplifier or regular fiber in a Raman pumping scheme, useful power output dictates the number of pumps required to reach a required pumping level and/or the distance between pump/fiber amplification stages.
Under current technology, the typical application for pump lasers is fiber amplification systems that utilize rare-earth doped fiber as the gain fiber. These gain fibers are located at attenuation-dictated distances along the fiber link. They typically are comprised of erbium-doped fiber amplifiers (EDFA). The laser pumps typically operate at 980 nanometers (nm) or 1480 nm, which correspond to the location of absorption peaks for the EDFA""s in the optical spectrum.
More recently, Raman pumping schemes have been proposed. The advantage is that special, periodic, EDFA amplifier gain fiber is not required to be spliced into the fiber link. Instead, regular fiber can be used. The result is a gain spectrum that is even wider than systems relying on EDFA""s. The bandwidth typically extends over the entire transmission bandwidth for fiber, stretching from 1250 to 1650 nm for some fiber compositions. The pump lasers are designed to operate in the wavelength range of 1060 to 1500 nm in the typical implementation.
Advantages associated with Raman amplification systems surrounds the fact that there is no longer a 3dB noise penalty associated with each amplifier, as occurs with EDFA""s. Raman amplification, however, is a non-linear process. As a result, relatively high pump powers are required.
In any case, high power pumps are required, regardless of whether EDFA""s or regular/Raman systems are used. Currently, pumps yielding 180 to 200 milliWatts (mW) of power are available. Newer system designs are requiring even higher power pumps, however.
As higher pump powers are required, additional optimizations are pursued in the pump laser module. One subject for these optimizations concerns the laser pump chip within the module. Specifically, in the present invention, the ridge profile of the laser chip is optimized both for high output power and also operation as a pump device, in which signal-band light, at other then the pump wavelength, is present.
Specifically, the present invention concerns a ridge waveguide laser module in which the ridge is flared in the direction of the rear or reflecting facet. Such a ridge profile yields a number of advantages depending on the specific implementation. The resulting increase in area of the rear facet spreads heat, to improve device performance and avoid catastrophic optical mirror damage (COD). It also lowers the power density for the same output intensity. Further, it contributes to the single lateral mode property, acting as a mode filter. Higher order modes suffer higher losses, which thus tends undermine the establishment of those modes. Further, with less light fed-back into the cavity from the rear facet, the laser device as a whole has reduced outside feedback sensitivity. Thus, in the implementation as a pump laser, less light at the signal wavelength returns to be amplified in a majority of the pump""s ridge waveguide cavity. Still further, any back facet power monitor can be moved to improve its sensitivity level due to the large output and input area.
In general, according to one aspect, the present invention concerns a ridge waveguide pump laser module. The module is adapted to generate light in the 1.2 to 1.6 micrometer wavelength range. It comprises a ridge waveguide laser chip that has a back, reflective facet and a front facet. The pump light is emitted through this front facet. According to the invention, the ridge width increases in the direction of the rear facet.
In specific implementations, a medial to front section of the ridge has a width of 1.5 to 7 micrometers. The rearward section of the ridge has a widest width of 3-14 micrometers. Specifically, in a preferred embodiment, the rearward section of the ridge has a maximum width of 8-9 micrometers and a medial to front section of the ridge has a width of 5 to 6 micrometers.
According to other specifics of the embodiments, the end of the optical fiber pigtail, which is positioned to receive light generated by the ridge waveguide laser chip, preferably comprises a flat-top wedge shaped fiber lens. The core of the fiber may also be flared in the direction of the chip and/or elliptical. Polarization maintaining fiber pigtails, such as polarization controlling fiber pigtails, are preferred especially if a grating is included for power stabilization.
In the anticipated implementation, the module is used to pump an EDFA amplifier. Alternatively, non-rare-earth doped gain fiber is used as the gain fiber in a Raman amplification scheme, however.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.